The invention relates to a fastener driving device and, more particularly, to a pneumatically actuated fastener driving device having a sequentially operated head valve that controls the loss of compressed air during the drive stroke and during the exhaust cycle.
A typical pressure operating fastener driving device includes a portable housing defining a guide track, a magazine assembly for feeding successive fasteners laterally into the guide track, a fastener driving element slidable in the drive track, a piston and cylinder arrangement for moving the fastener driving element through a cycle which includes a drive stroke and a return stroke, a main valve assembly for controlling communication of the cylinder with air under pressure communicated with the device and with the atmosphere to affect the cycling, and a manually operable valve for controlling the main valve assembly through pilot pressure.
A commonly used main valve assembly includes a one-piece valve member movable between two limiting positions. In one position of the valve member, the cylinder inlet is closed and the exhaust port is opened while in the other position, the cylinder inlet is open while the exhaust port is closed. In operation, the drive stroke is initiated by moving the valve member from its inlet closing position toward its inlet opening position. In the one-piece valve member arrangement, optimum communication of the driving pressure with the piston is obtained since such communication begins with the beginning of the movement of the valve member. However, closing of the exhaust port does not occur until movement of the valve member is completed, which may take a finite amount of time. During the return stroke, the exhaust port is opened initially and the inlet is not closed until valve member movement is completed, which again requires a finite amount of time. Consequently, it is well known that, due to the less rapid inlet closing movement of the valve member, some pressure is lost through the opening of the exhaust port before the inlet is closed. Air loss can be appreciable when the tool is used at high speed resulting in lower energy in succeeding cycles.
It is known that the air losses discussed above can be eliminated by using a main valve having separate inlet and exhaust valve members which are moved in sequence. Conventional arrangements of this type of main valve will close the exhaust before opening the inlet member during the drive stroke, thus eliminating the condition which caused the air losses discussed above. One such conventional driving device having a main valve which is sequentially operated was disclosed in U.S. Pat. No. 5,085,126 to Mukoyama. The main valve includes a first valve member which is movable from a first position which prevents communication of a main air reservoir with a piston chamber to a second position which permits communication between the main air reservoir and the piston chamber. The main valve further includes a second valve member which is movable from a first position communicating the piston chamber with an exhaust port to a second position which prevents the piston chamber from communicating with the exhaust port. The sequential movement of this main valve is mechanically accomplished. Thus, the first valve member moves in response to a pressure change established in a pilot pressure chamber. The first valve member thereafter engages the second valve member so as to move the second valve member to its second position to permit reservoir pressure to enter a piston chamber to drive the piston. Although the device of Mukoyama ensures positive sequential movement of the main valve, continuous operation of the device may cause wear due to the contact of the first valve member with the second valve member which may ultimately result in damage to the main valve.